Can I mix Flex-Tec by volume and not by weight?

No.

Flex-Tec is meant to by mixed by weight and not by volume.

The density of the two parts is different so the weight of equal volumes will not be the same. For correct usage and best results, we do recommend mixing the Flex-Tec by weight and not by volume.

We do have inexpensive scales, such as the Ohaus Valor 1000, available capable of mixing larger amounts of material that we highly recommend using if you are using Flex-Tec.